Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be a valued process with high commercial relevance for the preparation of low density polyethylene (LDPE).
A normal set-up of a plant for polymerizing or copolymerizing ethylenically unsaturated monomers such as ethylene and esters of vinyl alcohol in the presence of free-radical polymerization initiators consists essentially of a set of two compressors, a low-pressure and a high-pressure compressor, a polymerization reactor, which can be an autoclave or a tubular reactor or a combination of such reactors, and two separators for separating the monomer-polymer mixture leaving the reactor, wherein in the first separator, the high-pressure separator, the ethylene and comonomers separated from the monomer-polymer mixture are recycled to the reaction mixture between the low-pressure compressor and the high-pressure compressor, and the ethylene and comonomers separated from the mixture in the second separator, the low-pressure separator, are fed to the low-pressure compressor where it is compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the combined streams are further pressurized to the pressure of the high-pressure gas recycle stream. Such a high-pressure polymerization unit normally further includes apparatuses like extruders and granulators for pelletizing the obtained polymer. In case of tubular reactors, monomer supply to the reactor can either be carried out solely in the beginning of the reactor or only partly in the beginning with the other part fed via one or more side feed entries. Moreover, it is also common to introduce initiator in more than one place down the tube, thus creating more than one reaction zone.
Radically initiated high-pressure polymerization is an appropriated method for producing copolymers of ethylene and esters of vinyl alcohol, especially for preparing copolymers of ethylene and esters of vinyl alcohol having a vinyl ester content in the range of from 1% of weight to 45% of weight. Suitable reactors can be stirred autoclave reactors or tubular reactors. Such high-pressure polymerizations are for example described in U.S. Pat. No. 4,091,200 and WO 99/014251 A1.
A common feature for continuously operated polymerization processes, in which non-polymerized monomers are recycled to the polymerization process, is that continuously small amounts of the reaction mixture are discarded or purged from the process to avoid that impurities or inert materials, which are introduced with the raw materials or which form during the polymerization, accumulate in the process. Regarding processes for radically polymerizing ethylene and vinyl esters, EP 012 368 A1 for example describes a process in which gas, mainly ethylene, is taken from the high-pressure recycle stream, passed through a low temperature separator and discharged as off-gas. To be able to operate the polymerization process economically, there is of course the desired to minimize the amount of discarded material and to find ways for reutilizing the discarded materials meaningfully.
In homopolymerizing ethylene in high-pressure processes, it is common practice, if locally possible, to transfer the purged reaction gas back to an ethylene producing or converting unit such as a steam cracker for reprocessing. However, if the purged reaction gas contains higher amounts of comonomers such as esters of vinyl alcohol, it can be that the raw gas washers in the cracker are no longer able to fully remove these comonomers and they pass into the organic phase, in which they are not only impurities but also may degrade into products such as acidic acid which have a high corrosiveness.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and to provide a process which efficiently and economically separates esters of vinyl alcohol from a gas streams comprising ethylene and esters of vinyl alcohol and allows to transfer the off-gas stream from ethylene-vinyl ester polymerizations with no or only a minor content of esters of vinyl alcohol to an ethylene producing or converting unit and to take the off-gas from the high-pressure polymerization plant at a position of relatively low pressure.